Johnny Galecki as Leonard Hofstadter: An experimental physicist with an IQ of 173, who received his PhD when he was 24 years old. Leonard is a nerd who loves video games, comic books, and Dungeons & Dragons. Leonard is the straight man of the series, in which he shares an apartment in Pasadena, CA, with Sheldon. Leonard is smitten with his new neighbor Penny when they first meet, and they eventually marry.
Kunal Nayyar as Rajesh Koothrappali: A particle astrophysicist originally from New Delhi, India. Raj is initially unable to talk to women unless he drinks alcohol. Raj also has very feminine tastes and often takes on a stereotypical female role in his friendship with Howard as well as in the group of four men. Raj later dates Lucy (Kate Micucci), who also suffers from social anxiety, but it eventually ends; he later speaks to Penny without alcohol and thus overcomes his selective mutism. He begins dating Emily and their relationship later becomes exclusive. Raj also has a Yorkshire Terrier named Cinnamon.
"The Skank Reflex Analysis" "The Infestation Hypothesis" "The Wiggly Finger Catalyst" "The Russian Rocket Reaction" "The Rhinitis Revelation" "The Good Guy Fluctuation" "The Isolation Permutation" "The Flaming Spittoon Acquisition" "The Shiny Trinket Maneuver" "The Recombination Hypothesis" "The Beta Test Initiation" "The Werewolf Transformation" "The Hawking Excitation" "The Stag Convergence" "The Countdown Reflection"
"The Locomotive Manipulation" is the first episode in which Sheldon initiates a kiss with Amy. Although initially done in a fit of sarcasm, he discovers that he enjoys the feeling. Consequently, Sheldon slowly starts to open up over the rest of the season, and starts a more intimate relationship with Amy. However, in the season finale, Sheldon leaves temporarily to cope with several changes and Amy becomes distraught. However, in "The Prom Equivalency", he hides in his room to avoid going to a mock prom reenactment with her. In the resulting stand-off, Amy is about to confess that she loves Sheldon, but he surprises her by saying that he loves her too. This prompts Amy to have a panic attack.
After Sheldon has a food truck moved away from the building, Leonard and Penny are shocked to find he is the leader and sole member of the Tenants' Association for the building. Amy doesn't want to be caught in the middle but secretly suggests they rally support from the other tenants to vote Sheldon out. Nobody else supports Leonard so Amy tells them Sheldon was never added to the lease when he moved across the hall with her. Sheldon fires back with his own legal technicality so Amy is forced to support Leonard. She makes Sheldon happy by saying he can be Leonard's official opposition, making Leonard believe he made a mistake. Meanwhile, Howard and Raj find a drone. Raj returns it to the cute owner Cynthia and gets her number. Unfortunately, Cynthia watches footage of Raj on the drone and is immediately turned off.
Filming proceeds smoothly – makeup artists do touch-ups, a prop master replenishes the pickle supply – with humor and camaraderie between takes. When a reshoot is needed because Simon Helberg, who plays Howard, held his napkin in the wrong hand, Kaley Cuoco, who plays Penny, lifts her food box high and mockingly asks, “Does it go over my head? I don’t remember.”
Howard dresses as Sheldon for Halloween at work, hurting Sheldon's feelings. When Howard does not apologize, Amy and Sheldon dress as Bernadette and Howard at Penny and Leonard's party, angering Bernadette. Sheldon and Bernadette later bond over their painful childhood memories. Leonard is upset that Penny does not seem to remember that their first kiss was on Halloween. Penny later admits she does remember, but does not like to think of their first kiss as an unromantic drunken hookup. She likes to imagine their first kiss was one they shared on Leonard's birthday, which pleases him.
"The Plagiarism Schism" -- Pictured: Howard Wolowitz (Simon Helberg) and Bernadette (Melissa Rauch). Kripke (John Ross Bowie) has proof that Dr. Pemberton (Sean Astin) plagiarized his thesis in college, and Sheldon and Amy aren\'t sure if they should turn him in or not. Also, Wolowitz is happily surprised to learn that Bernadette wasn\'t the only waitress at the Cheesecake Factory who had a crush on him back in the day, on THE BIG BANG THEORY, Thursday, May 2 (8:00-8:31 PM, ET/PT) on the CBS Television Network. Photo: Michael Yarish/Warner Bros. Entertainment Inc. ÃÂ© 2019 WBEI. All rights reserved. (Photo: Michael Yarish, Warner Bros.)
After Sheldon reminds the guys about the time they mined Bitcoin without him, the guys go on a quest to find their suddenly valuable missing cryptocurrency. They realize the currency is on a laptop which Leonard gave to Penny when they dating, which she then gave to her ex-boyfriend Zack. After Leonard and Penny go to Zack's apartment, he shows them a seven-year-old video that Penny recorded on the laptop soon after she broke up with Leonard. Leonard is touched by the video which shows a drunk Penny lamenting the break-up of her relationship with him.